This invention relates to a metal gasket mainly suitable for use in internal combustion engines for automobiles.
FIG. 1 is a sectional view illustrating the proximity of a cooling water opening of an internal combustion engine using a prior art metal gasket. The metal gasket is clamped between a cylinder head 1 having a cooling water opening 1a and a cylinder 2 having a cooling water opening 2a. The metal gasket comprises a main plate 3 directly contacting the cylinder head 1, a main plate 4 directly contacting the cylinder 2 and a sub-plate 5 interposed between the main plate 3 and 4. Reference numeral 6 denotes a cooling water opening provided in the metal gasket made of the main plates 3 and 4 and the sub-plate 5.
In the prior art, the cooling water openings formed in the plates 3, 4 and 5 are the same in size and completely aligned with each other. Areas of the cooling water openings of the plates 3, 4 and 5 are usually smaller than those of the cooling water openings 1a and 2a of the cylinder head 1 and the cylinder 2. Such smaller areas of the cooling water openings of the gasket plates are set for suitably controlling flow rate of a cooling water flowing therethrough.
With the metal gasket shown in FIG. 1, however, surfaces 3a and 4a of the main plates 3 and 4 about the cooling water opening 6 are exposed to the cooling water in the cooling water openings 1a and 2a. Therefore, the exposed portions of the gasket are likely to be damaged in use for a long period of time.
In more detail, as the upper surface of the main plate 3 and the lower surface of the main plate 4 are directly contacted with the cylinder head 1 and the cylinder 2, the contacted surfaces of the gasket are frequently coated with various kinds of sealing agents including rubber or fluorine in order to improve sealing faculty. In this case, when the gasket has been used for long time, the coating layers of the sealing agents are peeled and circulated together with the cooling water. As a result, the peeled agents are sometimes attached to thermostats and radiators to cause engine troubles such as overheating.